LAS VEGAS -- When featherweight contender Joseph Diaz Jr., known to most as "Jo Jo," signed with Golden Boy Promotions following his run as a 2012 U.S. Olympian, the goal was clear: Put in the time, learn the craft as a pro, work hard and eventually get in position to win a coveted world title.

Diaz is almost there.

He has put in the time and effort. He has fought on small shows. He has worked his way up to fighting on bigger cards. He has fought on HBO and on Canelo Alvarez HBO PPV undercards.

Now Diaz is poised for the biggest fight of his career, a world title elimination fight against late substitute Rafael Rivera in the co-feature of the Alvarez-Gennady Golovkin fight, one of the most anticipated in years, on Saturday (HBO PPV, 8 p.m. ET) at T-Mobile Arena.

Diaz was initially supposed to face Jorge Lara but he withdrew on Tuesday after claiming a back injury.

He was replaced hours later by Rivera, who had been training for a fight next week.

Diaz, a 24-year-old southpaw from South El Monte, California, couldn't care less who he fights. He'd fight Godzilla for the opportunity to get a world title shot.

"It's been a long journey, man. It's been a lot of sacrifices, dedication and determination," Diaz told ESPN on Tuesday.

He was excited as could be talking about his fight with a reporter while his parents, off to the side, watched proudly.

"We started from the bottom. We started fighting (on small shows) at the Belasco Theater, on smaller networks, worked my to fighting on HBO and now I'm fighting on Canelo's undercards and here I am today fighting on the GGG-Canelo card," he said, talking a mile a minute. "And to be the in the co-main event is a dream come true. It's a blessing. I know why I'm here and I know what I'm capable of doing. I had a successful training camp and come Saturday night I'm gonna come out victorious."

Manager Ralph Heredia is proud and pleased with Diaz's progress.

"Every dream of a manager or promoter is to get a young prospect to this stage," he said. "The journey has been unbelievable. He's one of the easiest guys I've ever dealt with. Why? Because he strives to get better after each fight so that makes my job easier."

The winner of Saturday's fight will earn a mandatory shot at 126-pound world titleholder Gary Russell Jr. (28-1, 17 KOs).

Eric Gomez, president of Golden Boy Promotions, has watched Diaz develop since his pro debut in December 2012 and believes he is ready for anyone in the division.

"It's time. We've gotten him all the experience he needs," Gomez said. "We've put him in some really tough fights early on and he's got to the point now where he's ready to challenge. He's not a baby anymore. He's not a kid. You can see it in his body.

"Now is the time to challenge for the world title, and that's exactly what he's been asking us for," Gomez continued. "The first step toward the world title is Saturday. He wins that, then he's the mandatory. He's a good kid, but don't get mistaken, he's a killer in that ring. He's been in some dog fights. Saturday is no different. We've done our job as his promoter to get him the necessary experience, the necessary rounds, to be able to achieve a victory on Saturday night and it's up to him now."

Diaz has gained experience defeating solid opponents, such as former featherweight world title challenger Jayson Velez, Andrew Cancio, Horacio Garcia and then-undefeated Manuel Avila in May on the Alvarez-Julio Cesar Chavez Jr. HBO PPV undercard.

"I knew the road to getting a world title shot was not going to be an easy road," Diaz said. "I told my manager right before we even signed a professional contract and with Golden Boy -- I told him that I wanted to have tough fights before I even fight for a world title because I know if I fight tough fights, I'll be more prepared and experienced when I do fight for that world title. We know what we are capable of doing."

Diaz said the path he has taken to the precipice of a title shot is just how he envisioned his career going.

"It's exactly what we planned for," Diaz said. "I've been enjoying the ride and enjoying all the experiences that I've enduring. It's just exactly as planned. We're fighting tough fights, we're getting great exposure fighting on big networks and big cards and now I'm fighting on the biggest card. It's my coming out party, and I need to make a statement on Saturday night."

While most fighters would not want to look ahead to a title fight, Diaz said he has indeed allowed himself to think about the prospect of facing Russell.

"I think after this fight I think I might be a headliner fighting for that world title against Gary Russell," Diaz said. "It brings me a lot of excitement, fulfillment and I just cant wait to go out there and perform. I'm real excited. Come Saturday night, I want to showcase to everybody that I am the next superstar in boxing. I want to show them that I have the speed, that I have the charisma, I have the talent and I have the ability to overall become a superstar in boxing."

"I do allow myself to look at that because that's motivation for me when I'm in training camp," Diaz continued. "I know Gary Russell. He's a tremendous fighter. He has the quick hand speed. He has a vicious right hook. But at the end of the day, once we get past this fight, we'll focus on coming in with the right game plan with Gary Russell, focus on delivering the body shots. We've seen other fights Gary has fought before. We've seen his flaws and what can land during the fight. We know what to expect from Gary Russell.